In a shift control apparatus for controlling shifting actions of an automatic transmission which have a plurality of speed positions having respective different speed ratios and which are established by selectively engaging a plurality of frictional coupling devices, a second shifting-action decision to effect a shifting action of the automatic transmission to a given speed position may be made during a shifting control of the automatic transmission according to a first shifting-action decision to effect a shifting action to another speed position. This second shifting-action decision by the shift control apparatus takes place as a result of an operation by an operator of a vehicle such as an abrupt operation of an accelerator pedal or successive operations of a shift lever, or due to a change of a running state of the vehicle such as running on an uphill or slipping of the vehicle.
When the second shifting-action decision is made during the shifting control according to the first shifting-action decision, the shifting control according to the first shifting-action decision is stopped, and a shifting control to shift the automatic transmission to the target speed position selected according to the second shifting-action decision is immediately initiated. Patent Document 1 discloses an example of a shift control apparatus arranged to control the automatic transmission in such a manner. This shift control apparatus requires a shorter time for controlling the shifting actions of the automatic transmission, than a shift control apparatus arranged to initiate the shift control according to the second shifting-action decision after completion of the shifting control according to the first shifting-action decision.
However, there are cases where the shifting control according to the second shifting-action decision is not or cannot be initiated immediately after the second shifting-action decision made during the shifting control according to the first shifting-action decision. That is, the shifting control to shift the automatic transmission to the target speed position newly selected according to the second shifting-action decision cannot be initiated immediately after stopping of the shifting control according to the first shifting-action decision, if the number of the frictional coupling devices that should be concurrently controlled to establish the newly selected target speed position is larger than a predetermined upper limit number of the frictional coupling devices that can be concurrently controlled by the shift control apparatus.
For example, the shift control apparatus disclosed in the above-indicated Patent Document 1 is arranged to stop the first shifting control and immediately initiate the second shifting control if the second shifting-action decision is made before a moment of initiation of an actual engaging action of each frictional coupling device in the first shifting control, but continue the first shifting action and initiate the second shifting action after completion of the first shifting control if the second shifting-action decision is made after the above-indicated moment of initiation. If the second shifting control is initiated immediately after stopping of the first shifting control after the moment of initiation of the actual engaging action of the frictional coupling device in the first shifting control, this frictional coupling device whose engaging action has already been initiated must be released again, and another frictional coupling device must be engaged. In this case, therefore, it is difficult to control the shifting actions without a shifting shock.
[Patent Document 1]: JP-10-281277 A
Where the shifting control according to the second shifting-action decision cannot be initiated immediately after the moment of the second shifting-action decision during the shifting control according to the first shifting-action decision, as described above, the shift control apparatus disclosed in the above-identified Patent Document 1 is arranged to first establish the target speed position which is selected according to the first shifting-action decision and which is different from the target speed position selected according to the second shifting-action decision. Thus, the speed position to be once established before initiation of the shifting control according to the second shifting-action decision is fixed and kept unchanged, giving rise to a problem of insufficiency of the shifting control, such as insufficient reduction of the shifting shock and insufficient shortening of the required shifting time.
The present invention was made in view of the background art discussed above. It is therefore an object of the present invention to provide a shift control apparatus for an automatic transmission, which provides an improvement in the shifting control where it is impossible to effect the shifting control to shift the automatic transmission directly to the target speed position according to the second shifting-action decision.